The present invention relates to a multiuser receiver that performs coherent detection by despreading the signals received from a large number of users, including not only users to each of whom a single spreading code is assigned but also users who use multicode transmission with each user assigned a plurality of spreading codes, such as a receiver provided at a base station in a DS-CDMA (direct-spread code division multiple access) mobile communication system. The invention also relates to a communication system that performs multicode transmission.
Digital cellular wireless communication systems using DS-CDMA technology are being developed as next generation mobile communication systems that achieve wireless multimedia communications. CDMA is a system that performs simultaneous communications by assigning channels using codes, and it is said that CDMA provides an access method that is suitable for mobile multimedia communications since it can achieve high-speed communications by performing multicode transmission in which each user uses a plurality of channels.
A mobile communication system using DS-CDMA employs a coherent detection method in which a pilot signal or pilot and control information is transmitted over a channel different from the channels used for user data transmission, and the receiving end estimates the condition of the propagation path based on the received signal or information, and corrects the signal phase of user data based on the estimated value for the propagation path.
In the case of single-code transmission in which a single spreading code is assigned to each user, the pilot signal and data signal from the user are mapped to I-phase and Q-phase components, respectively, in QPSK demodulation. In the case of multicode transmission, one channel of pilot signal and a plurality of channels of data signal are transmitted from each user. It is required that a base station accommodate these users in a dynamic and flexible manner with a minimum amount of circuitry. In particular, in a configuration that employs an interference canceller such as the one disclosed in Japanese Unexamined Patent Publication No. 10-51353, as it is more effective to perform interference cancellation by preferentially selecting higher-speed users, user signal demodulation circuits for generating interference replicas are required to be able to serve as many high-speed users as possible by using a limited amount of circuitry. For this purpose, provisions must be made so that as many high-speed users as possible can be assigned, according to changes in user using conditions, for processing to a limited number of despreading circuits and coherent detection circuits provided for interference replica generation.
Accordingly, it is an object of the present invention to provide a multiuser receiver that can flexibly accommodate a plurality of users differing in transmission speed, according to the conditions, and a communication system that performs multicode transmission.
According to the present invention, there is provided a multiuser receiver comprising a plurality of receiving units and capable of assigning an arbitrary number of receiving units to each user in a dynamic and flexible manner in order to achieve reception from a plurality of users transmitting at different transmission speeds, wherein each of the plurality of receiving units includes: first and second despreaders each for despreading a received signal by a despreading code; a propagation path estimator which, when the despreading code used in the first despreader is a code for despreading a pilot signal, determines and outputs a propagation path estimated value based on the pilot signal output from the first despreader; and a first phase corrector which, based on the propagation path estimated value, corrects the phase of the despread received signal output from the second despreader, and wherein each of at least some of the plurality of receiving units further includes: a first selector which selects either the propagation path estimated value determined by the propagation path estimator contained in the same receiving unit or the propagation path estimated value determined by the propagation path estimator contained in another receiving unit, and supplies the selected propagation path estimated value to the first phase corrector; and a second phase corrector which corrects the phase of the despread received signal output from the first despreader by using the propagation path estimated value supplied from that other receiving unit and selected by the first selector.
According to the present invention, there is also provided a communication system that performs multicode transmission in which a plurality of channels that can be regarded as being equal in terms of propagation path effects are assigned to a transmitter and a receiver according to a bandwidth of transmit information, wherein the transmitter transmits a known signal for propagation path estimation by using either one of in-phase and quadrature components on one of the plurality of channels, and transmits the transmit information by using the other one of the components on the other channels, and the receiver performs propagation path estimation from the known signal and, using results of the propagation path estimation, coherently detects the channels carrying the transmit information.